Lithosphere

New in Lithosphere: Mars, Iraq, Canada, and the Spanish Pyrenees

Boulder, Colo., USA – Lithosphere science posted online 4 June 2012 includes a study of the Valles Marineris fault zone, Mars, and asks why such a trough system occurs there, when such structures on Earth are mainly associated with plate tectonics. Other papers discuss landslides in the Pyrenees; first evidence of a "missing" Cretaceous arc assemblage in the Iraqi segment of the Zagros orogenic belt; and new information on the age of the Okanagan Valley shear zone, Canada.

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This paper by F. Gutierrez and colleagues documents the largest active landslide in the Pyrenees and illustrates approaches that may be used to differentiate between gravitational faults and tectonic faults capable of producing damaging earthquakes. This is a relevant issue in many countries like the USA, where there are a number of active faults related to interstratal dissolution of evaporites and rock spread (e.g. Colorado, Utah, and Texas).

***************Recognition of Late Cretaceous Hasanbag ophiolite-arc rocks in the Kurdistan Region of the Iraqi Zagros suture zone: A missing link in the paleogeography of the closing Neotethys Ocean
S.A. Ali et al., University of Wollongong, School of Earth and Environmental Sciences, Northfields Avenue, Wollongong, NSW 2522, Australia. Posted online 4 June 2012; doi: 10.1130/L207.1.

The paper by S.A. Ali of the University of Wollongong and colleagues is (a) innovative because it provides first evidence of a "missing" Cretaceous arc assemblage in the Iraqi segment of the Zagros orogenic belt; (b) provocative because it challenges current ideas concerning Zagros evolution and anatomy; and (c) timely because there is much current literature on the neighboring Iranian segment of the Zagros orogen, whereas new information from Iraq is lacking.

The Okanagan Valley shear zone is a major extensional structure in the southern Canadian Cordillera that accommodated exhumation of the Shuswap metamorphic complex in the Eocene. New U/Pb dating of the rocks within the shear zone, including the Okanagan gneiss, reveals that exhumation occurred between 56 and 49 million years ago, concurrent with deformation and in situ melting; rocks were exhumed from about 20 km deep during this time, suggesting lateral stretching (east to west) of the crust of up to about 90 km. These new dates reveal that the Okanagan gneiss protolith (i.e. the rocks that were metamorphosed to form the gneiss) were sedimentary rocks intruded by igneous rocks around 160 Ma; this is contrary to earlier studies that suggested that the Okanagan gneiss was an exposed outlier of Proterozoic North American basement. This study by S.R. Brown and colleagues confirms the presence of a major Cordilleran structure and accurately establishes its age and magnitude.

Despite four decades of research, the origin of the longest known trough system in the solar system, Valles Marineris on Mars, remains uncertain. Its formation mechanism has been variably related to rifting, strike-slip faulting, and subsurface mass removal. This study by UCLA scientist An Yin focuses on the structural geology of Ius and Coprates Chasmata using THEMIS (Thermal Emission Imaging System), Context Camera (CTX), and HiRISE (High Resolution Imaging Science Experiment) images. The main result of the work is that the troughs and their plateau margins have experienced left-slip transtensional deformation. The discovery of a large-scale (>2000 km in length and >100 km in slip) and rather narrow (<50 km in width) strike-slip fault zone by this study begs the question of why such a structure, typically associated with plate tectonics on Earth, has developed on Mars.